The present invention relates to an abrasive machine, more precisely relates to an abrasive machine, which sandwiches a work piece between a lower plate and an upper plate and relatively moves the two plates so as to abrade the work piece therebetween.
Conventionally, a thin plate work piece, e.g., a silicon wafer, is abraded by a lapping machine and polished by a polishing machine.
In the conventional lapping machine, for example, an external gear, which acts as a sun gear, and an internal gear are rotated at different angular velocity, so that each carrier plate, which holds the work pieces in through-holes, is rotated on its axis and moved round. Each carrier plate acts as a planet gear. The work pieces held by the carrier plates are sandwiched between an upper plate and a lower plate, which are respectively provided on the upper side and the lower side of the carrier plates. An abrasive agent (slurry) is supplied to spaces between the work pieces and the upper and lower plates. The upper and lower plates are relatively moved (rotated and/or swung) with respect to the work pieces, so that an upper face and a lower face of the work pieces can be simultaneously lapped.
By using the lapping machine, the work pieces can be precisely flatly lapped. Further, by simultaneously lapping the both faces of the work piece, working efficiency can be improved. The lapping machine has been employed, for example, to lap silicon wafers for semiconductor chips.
The conventional polishing machine generally has a rotary polishing plate, whose upper face is covered with a polishing cloth, and a holding unit including a holding plate. The holding plate is rotatably provided above the polishing plate, movable in the vertical direction and capable of holding the work pieces on a bottom face. Note that, the polishing plate corresponds to the lower plate; the holding plate corresponds to the upper plate. In the conventional polishing machine, the slurry is supplied, and the polishing plate is relatively moved with respect to the work pieces to polish a surface of the work pieces like mirror faces.
An example of the conventional lapping machine will be explained with reference to FIG. 6.
A bottom face of an upper plate 20 is an abrasive face, which laps upper faces of work pieces 10 (silicon wafers). Keys 21 are provided on an upper face of the upper plate 20.
A cylinder unit 22, e.g., a hydraulic cylinder unit, is provided to an upper part of a gate-shaped frame 14. The upper plate 20 is connected to a lower end of a piston rod 22a of the cylinder unit 22 by a rotary plate 23 and connecting rods 27. With this structure, the upper plate 20 is rotatably suspended. A connecting section 22b is fixed to the rotary plate 23, the piston rods 22a is not rotatable, and the rotary plate 23 and the upper plate 20, which are mutually connected by the connecting rods 27, are rotatably connected to the piston rod 22b without falling therefrom. The upper plate 20 applies a pressing force, which is caused by weight of the upper plate 20, to a lower plate 30. The pressing force can be adjusted by controlling a lifting force of the cylinder unit 22.
The keys 21 of the upper plate 20 are engaged with key grooves of a rotary member 54, which is rotated by a motor 70, so the upper plate 20 is rotated by the motor 70. A shaft 54a is downwardly extended from a lower end of the rotary member 54. A gear 54b is fixed to a lower end of the shaft 54a, and an idle gear 63 is engaged with the gear 54b and a gear 64, which is fixed to a spindle 60. With this structure, power of the motor 70 is transmitted to the upper plate 20 via the rotary member 54. By connecting the upper plate 20 to the rotary member 54 with the keys 21, a wide space for maintenance and setting the work pieces 10 can be formed between the upper plate 20 and the lower plate 30.
An external gear 50 is engaged with carrier plates 40. A first hollow shaft 50a, which is coaxial to the rotary shaft 54a, is connected to the external gear 50. A gear 50b of the first hollow shaft 50a is engaged with a gear 65 of the spindle 60.
A second hollow shaft 30b, which is coaxial to the first hollow shaft 50a, is connected to the lower plate 30. A gear 30b, which is fixed to a mid part of the second hollow shaft 30a, is engaged with a gear 61 of the spindle 60.
An internal gear 52 is engaged with the carrier plates 40. A third hollow gear 52a, which is coaxial to the second hollow shaft 30a, is connected to the internal gear 52. A gear 52b of the third hollow shaft 52a is engaged with a gear 62 of the spindle 60.
The spindle 60 is connected to a reduction gear system 69, and the reduction gear system 69 is connected to the motor, e.g., an electric motor, an oil motor, by a belt.
With the above described structure, the power of the motor 70 is transmitted by the reduction gear system 69, the gears and the shafts, so that the upper plate 20, the lower plate 30, the external gear 50 and the internal gear 52 are rotated.
In the conventional lapping machine, the pressing force of the upper plate 20, which is caused by the weight of the upper plate 20 and which presses the lower plate 30, is adjusted by controlling the lifting force of the cylinder unit 22. If fluid pressure in a lower chamber of the cylinder unit is made higher, the piston rod 22a is retracted into the cylinder unit 22 and the pressing force of the upper plate 20, which presses the lower plate 30, can be reduced. Namely, the maximum pressing force of the upper plate 20 is equal to the weight of the upper plate 20.
When silicon wafers are lapped by the lapping machine, there are minute projections and holes in surfaces of the silicon wafers 10. Firstly, the fluid pressure in the lower chamber of the cylinder unit 22 is made high so as to reduce the pressing force of the upper plate 20. The surfaces of the silicon wafers 10 are lapped with lower pressing force. Then, the fluid pressure in the cylinder unit 22 is gradually reduced so as to gradually increase the pressing force of the upper plate 20. By this control, the surfaces of the silicon wafers 10 are smoothly lapped and the silicon wafers have uniform thickness. In this state, the pressing force can be uniformly applied to the whole surfaces of the silicon wafers 10. Then, the entire weight of the upper plate 20 is applied to the lower plate 30 as the pressing force. Adjusting the pressing force should be executed smoothly.
However, in the conventional lapping machine, the pressing force of the upper plate 20 is adjusted by changing the fluid pressure in the cylinder unit 22, so mechanical resistance in the cylinder unit 22 influences the adjustment. Namely, it is difficult to precisely and linearly control the pressing force. Namely, the pressing force of the upper plate 20 is directly changed according to tensile stress of the piston rod 22a, and the pressing force of the upper plate 20 is changed while a bottom lapping face of the upper plate 20 contacts the work pieces 10. Thus, a stroke of a piston (not shown) in the cylinder unit 22 is equal to the sum total of amount of lapping (abrading) the wafer 10 and minute elastic elongation of the piston rod 22a. Namely, it is very very short.
It is difficult to perfectly smoothly move the piston due to friction between the piston and an inner circumferential face of the cylinder unit 22, so the piston is braked in the stroke. This phenomenon is called knocking. Even if the braking action is minute, the stroke of the piston is very very short, so the non-smooth action of the piston influences the pressing force of the upper plate 20. Therefore, it is very difficult to smoothly adjust the pressing force of the upper plate 20. The piston usually slides on the inner circumferential face of the cylinder unit 22 together with sealing members (not shown). The sealing members also cause the friction.
Further, it is very difficult to make the length of the connecting rods 27 perfectly same. The upper plate 20 is slightly inclined by the minute difference of the length thereof. If the upper plate 20 is inclined, the pressing force partially concentrates, so that the work pieces 10 cannot be uniformly lapped. Especially, in the case of lapping the silicon wafers, the lapping accuracy is quite high, e.g., sub-micron order, so it is more difficult to precisely lap the silicon wafers by the conventional lapping machine. And, it is also very difficult to realize the lapping machine for lapping large wafers.